Silver halide photography has been practiced for more than a century. The radiation sensitive silver halide compositions initially employed for imaging were termed emulsions, since it was not originally appreciated that a solid phase was present. The term "photographic emulsion" has remained in use, although it has long been known that the radiation sensitive component is present in the form of dispersed microcrystals, typically referred to as grains.
Over the years silver halide grains have been the subject of intense investigation. Although high iodide silver halide grains, those containing at least 90 mole percent iodide, based on silver, are known and have been suggested for photographic applications, in practice photographic emulsions almost always contain silver halide grains comprised of bromide, chloride, or mixtures of chloride and bromide optionally containing minor amounts of iodide. Up to about 40 mole percent iodide, based on silver, can be accommodated in a silver bromide crystal structure without observation of a separate silver iodide phase. However, in practice silver halide emulsions rarely contain more than about 15 mole percent iodide, with iodide well below 10 mole percent being most common.
All silver halide grains, except high iodide silver halide grains, exhibit cubic crystal lattice structures. However, grains of cubic crystal lattice structures can differ markedly in appearance.
In one form silver halide grains when microscopically observed are cubic in appearance. A cubic grain 1 is shown in FIG. 1. The cubic grain is bounded by six identical crystal faces. In the photographic literature these crystal faces are usually referred to as {100} crystal faces, referring to the Miller index employed for designating crystal faces. While the {100} crystal face designation is most commonly employed in connection with silver halide grains, these same crystal faces are sometimes also referred to as {200} crystal faces, the difference in designation resulting from a difference in the definition of the basic unit of the crystal structure. Although the cubic crystal shape is readily visually identified in regular grains, in irregular grains cubic crystal faces are not always square. In grains of more complex shapes the presence of cubic crystal faces can be verified by a combination of visual inspection and the 90.degree. angle of intersection formed by adjacent cubic crystal faces.
The practical importance of the {100} crystal faces is that they present a unique surface arrangement of silver and halide ions, which in turn influences the grain surface reactions and adsorptions typically encountered in photographic applications. This unique surface arrangement of ions as theoretically hypothesized is schematically illustrated by FIG. 2, wherein the smaller spheres 2 represent silver ions while the larger spheres 3 designate bromine ions. Although on an enlarged scale, the relative size and position of the silver and bromide ions is accurately represented. When chloride ions are substituted for bromide ions, the relative arrangement would remain the same, although the chloride ions are smaller than the bromide ions. It can be seen that a plurality of parallel rows, indicated by lines 4, are present, each formed by alternating silver and bromine ions. In FIG. 2 a portion of the next tier of ions lying below the surface tier is shown to illustrate their relationship to the surface tier of ions.
In another form silver halide grains when microscopically observed are octahedral in appearance. An octahedral grain 5 is shown in FIG. 3. The octahedral grain is bounded by eight identical crystal faces. These crystal faces are referred to as {111} crystal faces. Although the octahedral crystal shape is readily visually identified in regular grains, in irregular grains octahedral crystal faces are not always triangular. In grains of more complex shapes the presence of octahedral crystal faces can be verified by a combination of visual inspection and the 109.5.degree. angle of intersection formed by adjacent octahedral crystal faces.
Ignoring possible ion adsorptions, octahedral crystal faces differ from cubic crystal faces in that the surface tier of ions can be theoretically hypothesized to consist entirely of silver ions or halide ions. FIG. 4 is a schematic illustration of a {111} crystal face, analogous to FIG. 2, wherein the smaller spheres 2 represent silver ions while the larger spheres 3 designate bromine ions. Although silver ions are shown at the surface in every available lattice position, it has been suggested that having silver ions in only every other available lattice position in the surface tier of atoms would be more compatible with surface charge neutrality. Instead of a surface tier of silver ions, the surface tier of ions could alternatively be bromide ions. The tier of ions immediately below the surface silver ions consists of bromide ions.
In comparing FIGS. 1 and 2 with FIGS. 3 and 4 it is important to bear in mind that both the cubic and octahedral grains have exactly the same cubic crystal lattice structure and thus exactly the same internal relationship of silver and halide ions. The two grains differ only in their surface crystal faces. Note that in the cubic crystal face of FIG. 2 each surface silver ion lies immediately adjacent five halide ions, whereas in FIG. 4 the silver ions at the octahedral crystal faces each lie immediately adjacent only three halide ions.
Much less common than either cubic or octahedral silver halide grains are rhombic dodecahedral silver halide grains. A rhombic dodecahedral grain 7 is shown in FIG. 5. The rhombic dodecahedral grain is bounded by twelve identical crystal faces. These crystal faces are referred to as {110} (or, less commonly in reference to silver halide grains, {220}) crystal faces. Although the rhombic dodecahedral crystal shape is readily visually identified in regular grains, in irregular grains rhombic dodecahedral crystal faces can vary in shape. In grains of more complex shapes the presence of rhombic dodecahedral crystal faces can be verified by a combination of visual inspection and measurement of the angle of intersection formed by adjacent crystal faces.
Rhombic dodecahedral crystal faces can be theoretically hypothesized to consist of alternate rows of silver ions and halide ions. FIG. 6 is a schematic illustration analogous to FIGS. 2 and 4, wherein it can be seen that the surface tier of ions is formed by repeating pairs of silver and bromide ion parallel rows, indicated by lines 8a and 8b, respectively. In FIG. 6 a portion of the next tier of ions lying below the surface tier is shown to illustrate their relationship to the surface tier of ions. Note that each surface silver ion lies immediately adjacent four halide ions.
Although photographic silver halide emulsions containing cubic crystal lattice structure grains are known which contain only regular cubic grains, such as the grain shown in FIG. 1, regular octahedral grains, such as the grain shown in FIG. 3, or, in rare instances, regular rhombic dodecahedral grains, such as the grain shown in FIG. 5, in practice many other varied grain shapes are also observed. For example, silver halide grains can be cubo-octahedral-that is, formed of a combination of cubic and octahedral crystal faces. This is illustrated in FIG. 7, wherein cubo-octahedral grains 9 and 10 are shown along with cubic grain 1 and octahedral grain 5. The cubo-octahedral grains have fourteen crystal faces, six cubic crystal faces and eight octahedral crystal faces. Analogous combinations of cubic and/or octahedral crystal faces and rhombic dodecahedral crystal faces are possible, though rarely encountered. Other grain shapes, such as tabular grains and rods, can be attributed to internal crystal irregularities, such as twin planes and screw dislocations. In most instances some corner or edge rounding due to solvent action is observed, and in some instances rounding is so pronounced that the grains are described as spherical.
It is known that for cubic crystal lattice structures can take any one of seven possible distinct crystallographic forms. However, for cubic crystal lattice structure silver halides only grains having {100} (cubic), {111} (octahedral), or rarely, {110} (rhombic dodecahedral) crystal faces, individually or in combination, have been identified.
It is thus apparent that the photographic art has been limited in the crystal faces presented by silver halide grains of cubic crystal lattice structure. As a result the art has been limited in modifying photographic properties to the choice of surface sensitizers and adsorbed addenda that are workable with available crystal faces, in most instances cubic and octahedral crystal faces. This has placed restrictions on the combinations of materials that can be employed for optimum photographic performance or dictated accepting less than optimum performance.